1. Field of the Invention
This invention relates to a radiation film, which comprises a substrate and emulsion layers overlaid on opposite surfaces of the substrate and is used by being sandwiched between two radiation intensifying screens. This invention also relates to a combination of a radiation film and radiation intensifying screens. This invention additionally relates to a radiation image read-out method for reading out radiation images, which have been recorded on a radiation film. This invention further relates to an energy subtraction processing method for radiation images.
2. Description of the Prior Art
Operations for recording radiation images are carried out in various fields. For example, radiation images to be used for medical purposes are recorded as in X-ray image recording for medical diagnoses. Also, radiation images to be used for industrial purposes are recorded as in radiation image recording for non-destructive inspection of substances. During such operations for recording radiation images, radiation films, e.g., X-ray films, and radiation intensifying screens are utilized. Specifically, a radiation intensifying screen is superposed upon one surface of a radiation film in close contact therewith. Alternatively, radiation intensifying screens are superposed upon opposite surfaces of a radiation film in close contact therewith.
Basically, a radiation film is composed of a substrate and an emulsion layer overlaid on one surface of the substrate or emulsion layers overlaid on opposite surfaces of the substrate. The emulsion layer is composed of a binder and a silver halide dispersed in the binder.
Also, basically, a radiation intensifying screen is composed of a substrate and a phosphor layer overlaid on one surface of the substrate. The phosphor layer is composed of a binder and phosphor grains dispersed in the binder. The phosphor grains produce fluorescence having a high luminance when they are excited by radiation, such as X-rays. Therefore, the phosphor produces the fluorescence having a high luminance in accordance with the amount of radiation, which has passed through an object. The radiation film, which is superposed upon the surface of the phosphor layer of the radiation intensifying screen in close contact therewith, is also exposed to the fluorescence produced by the phosphor. Therefore, a sufficient exposure of the radiation film can be achieved with a comparatively small dose of radiation.
In order for the radiation dose to the object to be kept as low as possible, the sensitivity of the imaging medium system, which is composed of the radiation intensifying screen and the radiation film and is utilized during the operation for recording a radiation image, should be as high as possible. Also, it is desired that the image thus obtained has good image quality, such as sharpness and granularity.
For the purposes of recording a radiation image with a high sensitivity, a double-faced emulsion film, which is composed of a substrate and emulsion layers overlaid on opposite surfaces of the substrate, has heretofore been employed as the radiation film. Radiation intensifying screens (i.e., a front screen and a back screen) are located on opposite sides of the radiation film, and an imaging medium system (i.e., a screen-film system) is thereby formed.
As the double-faced emulsion films, a regular type of film and an orthochromatic type of film are well known. Both of the emulsion layers overlaid on opposite surfaces of the regular type of film have a wavelength sensitivity to the blue optical region of approximately 350 nm to approximately 480 nm. Both of the emulsion layers overlaid on opposite surfaces of the orthochromatic type of film have a sensitivity to the green optical region of approximately 350 nm to 570 nm. Therefore, in cases where the regular type of double-faced emulsion film is used, screens for intensifying the blue region are utilized in combination with the film. In cases where the orthochromatic type of double-faced emulsion film is used, screens for intensifying the green region are utilized in combination with the film.
The image obtained with the radiation film described above is primarily an uncolored variable density image. For the purposes of improving diagnostic efficiency and accuracy, or the like, a color radiograph, which expresses image information in pseudo colors, is often utilized. The color radiograph can be obtained with a direct process, in which a color film is combined with color intensifying screens (capable of emitting blue light, green light, and red light), or with an indirect process, in which different levels of density in the conventional monochromatic photograph are indicated by different colors. Such processes are described in, for example, Japanese Patent Publication No. 48(1973)-12676.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
In the diagnostic fields using radiation, or the like, energy subtraction processing is often carried out by using stimulable phosphor sheets. With energy subtraction processing, an image of a specific object is extracted from radiation images which have been stored on stimulable phosphor sheets. The extracted image is utilized for making various diagnoses, or the like.
Specifically, with energy subtraction processing, each of at least two stimulable phosphor sheets is exposed to one of at least two kinds of radiation, which have different energy distributions and have passed through an object constituted of bones and soft tissues, and radiation images of the object are thereby recorded on the stimulable phosphor sheets. Each of the stimulable phosphor sheets is thereafter exposed to stimulating rays, and each of the radiation images is photoelectrically detected and converted into a digital image signal made up of a series of image signal components representing each radiation image. The image signal components of the digital image signals thus obtained, which image signal components represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of only the bones or only the soft tissues represented by the radiation images. A method for obtaining an energy subtraction image is disclosed in, for example, U.S. Pat. No. 4,855,598.
A method for obtaining an energy subtraction image by reading out X-ray images, which have been recorded on X-ray films, by using a film digitizer has also been proposed in, for example, Japanese Unexamined Patent Publication No. 59(1984)-83147.
An object, such as a human body, includes soft tissues and bones, which exhibit different radiation transmission characteristics for radiation having different energy distributions. Therefore, the energy distribution of radiation irradiated through the object is changed during the operations for recording radiation images, and images are thereby obtained which vary in contrast between the soft tissue patterns and the bone patterns. Energy subtraction processing is then carried out on the images, and an image representing only the soft tissue patterns or an image representing only the bone patterns is thereby obtained. Therefore, for the purposes of obtaining an image in which only the soft tissue patterns have been emphasized, or an image, in which only the bone patterns have been emphasized, it has heretofore been necessary to use two recording media, such as stimulable phosphor sheets or X-ray films.
Also, with energy subtraction processing described above, two images of an object are recorded on two recording media, and two kinds of image signals are obtained from the two recording media. A subtraction process is then carried out on the two kinds of image signals, and an image signal representing only a specific part of the object is thereby obtained. Therefore, when the subtraction process is carried out on the image signals, it is necessary to adjust the image signals such that the positions of the two images represented by the image signals may coincide with each other. Such adjustment is not easy to carry out. If the positions of the image signals do not coincide with each other during the subtraction process, the problems will occur in that an unsharp energy subtraction image is obtained.
Additionally, when a radiation image of an object is recorded on the radiation film having the emulsion layers overlaid on opposite surfaces, the fluorescence, which has been produced by a radiation intensifying screen located on the side outward from the radiation film, is directly absorbed by the adjacent emulsion layer of the radiation film. The fluorescence also passes through the adjacent emulsion layer (as cross-over light) and impinges upon the opposite emulsion layer. The cross-over light has a spread due to scattering during the passage through the emulsion layer, or the like. As a result, the problems occur in that the image quality of the radiation image obtained with the radiation film becomes bad (i.e., the radiation image becomes unsharp).